(Received 28 November 2013;accepted 3 December 2013;online 11 December 2013)

In the title mol­ecule, C48H42N4O5, a potential non-linear optical compound, the furan ring [r.m.s. deviation = 0.010 (1) Å] and the indolyl­idene ring system [r.m.s. deviation = 0.013 (2) Å] are inclined to one another by 18.52 (6)°. This is similar to the arrangement [16.51 (18)°] found for the N-hy­droxy­ethyl adduct of the title compound [Bhuiyan et al. (2011). Mol. Cryst. Liq. Cryst.548, 1–12]. Replacing the hy­droxy­ethyl group with 3,5-di­benzyl­oxybenzoate has not resulted in a non-centrosymmetric lattice arrangement or significant changes to the basic mol­ecular structure. In the crystal, mol­ecules are linked via pairs of C—H⋯N hydrogen bonds, forming inversion dimers with an R22(20) ring motif. The dimers are linked via C—H⋯O hydrogen bonds, forming C(17) chains along [010]. The chains are linked by further C—H⋯N hydrogen bonds, forming layers parallel to (001) and enclosing R22(44) ring motifs. There are also C—H⋯π inter­actions present, stabilizing the inter­layer orientation of the pendant bis­(benz­yloxy)benzo­yloxy group.

Organic non-linear optical (NLO) chromophores are highly polar and tend to readily form aggregates in both solution and/or the solid state (Smith et al., 2006). This is a potential downfall when considering the usage of NLO materials in a host polymer. The presence of aggregation will lower the overall poling efficiency and increase the tendency for relaxation of the aligned dipoles which decreases the observed macroscopic response. The introduction of bulky, arene-rich substituents has been shown to be very effective in reducing aggregation and increasing the observed NLO response (Kim et al., 2007). We report herein on the synthesis of an indoline chromophore which contains a 3,5-dibenzyloxybenzoate substituent and which was designed to reduce the tendency for molecular aggregation to occur.

The molecular structure of the title compound is shown in Fig. 1. The 5-membered ring plane of atoms (O1/C4—C7) of the acceptor group (hereafter CTF; 3-cyano-5,5-Dimethyl-2,5-dihydrofuran-2-ylidene]propanedinitrile) can be regarded as planar [r.m.s. deviations 0.010 (1) Å]. The dicyano group (N1/C1-C3/N2) is planar [r.m.s.d. 0.013 (2) Å] but twisted by 5.75 (10)° with respect to the CTF group; this is similar to the twist in the related compound (NOJKUT - see above) of 5.69 (17)°. We note that in the related compound (NAPZAH - see above) the subtended dicyano group is coplanar with the 1,3-thiazolylidene ring.

The fused indolylidene system (N4/C16-C23) is also essentially planar [r.m.s.d. 0.013 (2) Å] and makes a dihedral angle with the CTF ring of 18.52 (6)°, similar to the 16.51 (18)° angle found in the N-hydroxyethyl adduct of the title compound, 2-(3-cyano-4-{5-[1-(2-hydroxy-ethyl)-3,3-dimethyl-1,3-dihydro- indol-2-ylidene]-penta-1,3-dienyl}-5,5-dimethyl-5H-furan-2-ylidene)-malononitrile (henceforth FAFP; Bhuiyan et al., 2011). This angle reflects a twist in the C11–C14 polyene chain beginning at C11 and the plane through C11–C14 subtends 7.23 (13)° with the CTF plane; a view illustrating the relative conformations of the various chemical entities is given in Fig. 2. Again this is in contrast to the smaller NAPZAH structure where the polyene chain atoms and indolylidene ring are coplanar, and twist from the 5-membered 1,3-thiazolylidene ring plane by 5.48 (6)°. Rings A (C29–C34) and B (C43–C48) subtend an angle of 18.72 (7)°, whilst the phenyl ring C (C36–C41) makes an angle of 54.69 (8)° to ring A, and 65.37 (9)° to ring B. Ring A makes an angle of 44.54 (6)° to the indolylidene ring.

There is considerable delocalization of charge along the polyene/CTF chain with a bond length alternation (BLA) value of 0.016 Å compared with the free CTF value of 0.108 Å (Li et al., 2005) 0.060 Å in (GIMQAV - see above) and 0.024 Å in FAFP.

The crystal packing involves attractive non-classical hydrogen bond interactions of the (alkene)C—H···N(cyano), (phenyl)C—H···O and phenyl(C—H)···N(cyano) types (Table 1, Fig. 3). The alkene H15···N3 interaction (entry 1, Table 1) connects molecules around centers of symmetry (e.g. at 1/2, 1/2, 0) into dimer layers, approximately parallel to (1,-1,1) or (3,1,1) crystallographic planes, which can be described by the H bonding motif R22(20) (Bernstein et al., 1995). The other two main contacts (entries 2 and 3, Table 1) connect other molecules into these layers. The H30···O1 interaction forms a C(17) motif as it links the identical molecule related by a b axis translation. The H32···N2 interactions form a R22(44) motif utilizing an inversion center at (0, 1/2, 0). In addition, there are C—H···π interactions between methyl H9A and the phenyl ring (atoms C43—C48) which stabilizes the interlayer orientation of the "dangling" bis-benzyloxoy-benzoic acid moiety. Providing weak links between the layers are alkene C—H···N(cyano) interactions involving atoms C12 and C14, an interaction also observed previously in FAFP (Bhuiyan et al., 2011).

The title compound was synthesized by the procedure described by Clarke et al. (2009). Single crystals were grown by slow ether diffusion into an ethyl acetate solution of the title compound. Spectroscopic and other data for the title compound are included in the archived CIF.

Eight reflections affected by the backstop were omitted from the refinement. All the H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms: C—H = 0.98, 0.99 and 0.95 Å CH3, CH2 and CH(aromatic) H atoms, respectively, with Uiso(H) = 1.5Ueq(C-methyl) and = 1.2Ueq(C) for other H atoms. The methyl H atoms were allowed to rotate freely about the adjacent C—C bond.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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